1. Field of the Invention
This invention pertains to active position sensors such as motion, edge, frequency and speed sensors or counting devices. In particular, it relates to gear tooth sensors incorporating a Hall effect device and which sense the presence or absence of gear teeth or voids in a metallic member as the teeth or voids pass near a sensor device.
2. Description of the Prior Art
Prior art approaches to gear tooth sensors include magneto-resistive devices, proximity switches, and other similar devices. More recently, position sensors incorporating Hall effect devices have been developed which provide a convenient logic signal output when sensing metallic gear teeth or voids. Essentially, when a Hall effect device is placed in a magnetic field and oriented transversely to a current flowing through the device, a voltage output in direct proportion to the strength of the magnetic flux component at right angles to the Hall device is produced. Typically, the Hall device signal is supplied to a transistor and the voltage from the Hall device is used to switch the transistor on and off and produce a logic signal.
Many have developed position sensors which incorporate Hall devices. Most of the prior art devices are susceptible to changes in temperature and sensitive variances in the distance between the gear tooth and the sensor, commonly known as the air gap. One such device is shown in U.S. Pat. No. 4,745,363 to Carr, et al. The device shown in Carr, et al. incorporates spaced apart, offset flux concentrators to provide a sensitive device with favorable tolerances to temperature and air gap.
U.S. Pat. No. 4,293,814 to Boyer and U.S. Pat. No. 4,524,932 to Bodziak both disclose proximity or position sensing devices incorporating Hall effect sensors. Bodziak discloses a Hall effect sensor disposed on the north pole surface of a cubic magnet. The Bodziak magnet/Hall effect sensor assembly is disposed below the rail of a railroad track to sense the metallic wheels of railroad cars as they pass over or near the device. The Bodziak device produces a square wave output or pulse signal in response to detection of a railroad wheel.
The Boyer device operates in substantially a similar manner as the Bodziak device. Neither Bodziak nor Boyer disclose a pole piece disposed between the magnet and the Hall effect device of the sensor assembly. The circuit in Boyer is directed towards providing a self-adjusting threshold voltage for use with a Hall effect sensor. The output of the circuit shown in Boyer provides a square wave signal indicative of the position of a tone wheel with respect to the Hall effect device.
U.S. Pat. No. 4,518,918 to Avery discloses a dual Hall effect sensor wherein two Hall effect devices are mounted on a pole of a magnet and positioned so that one Hall device detects a valley or notch in the gear when simultaneously the other Hall device is detecting the protrusion or tooth of the gear or tone wheel. The outputs of the sensors are fed into a differential amplifier circuit in order to produce a pulse train signal which corresponds to the speed of rotation of the tone wheel.
Other patents disclosing position sensors which incorporate Hall effect devices activated in response to moving magnetic shunts or gear teeth are shown in U.S. Pat. No. 4,853,629 to Rops; U.S. Pat. No. 4,406,272 to Kiess, et al.; and U.S. Pat. No. 4,725,776 to Onodera, et al. U.S. Pat. No. 4,481,469 to Hauler, et al. discloses an alternate approach using a Hall effect device to detect gear teeth. The Hall effect device in the Hauler, et al. sensor is positioned so as to detect the tangential magnetic field rather than the radial magnetic field variations attributable to the rotating tone wheel. Other examples of proximity or position sensors incorporating Hall effect devices are disclosed in U.S. Pat. No. 3,195,941 to Burig, et al. and U.S. Pat. No. 4,859,941 to Higgs, et al.
Consistency of operation as well as ease of manufacture are primary objectives in the design of a position sensor. Various Hall effect devices packaged in plastic-encapsulated three terminal packages are available on the open market. These off-the-shelf devices typically have wide variances in operational characteristics with respect to temperature. In addition, the magnetic flux switch points of the Hall effect device are not always suitable for use in position sensor applications since magnets of corresponding magnetic strength do not provide sufficient flux deviations to operate the Hall devices appropriately over a broad range of conditions such as variable tooth width, height, and wide variances in the air gap requirements. With these thoughts in mind, a new position sensor design which includes a structure to limit and control magnetic flux impinging upon the Hall effect device and includes flux dispersion means for dispersing the magnetic flux so as to produce the widest flux differential in response to the passing of a tooth of a tone wheel and thereby increase sensitivity of the sensor is desired.